The International System, or SI (cont.)

Length
In 1960 an atomic standard for the meter was also established, using the wavelength of the orange-red light emitted by atoms of krypton(86-Kr) in a glow discharge tube. In November 1983 the length standard was changed again, in a more radical way. The new definition of the meter is the distance that light travels(in vacuum) in 1/299, 792,458 second. This has the effect of defining the speed of light to be precisely 299, 792, 458 m/s; we then define the meter to be consisten with this number and with the above definition of the second.This provides a much more precise standard of length than the one based on a wavelength of light.

Next:The International System, or SI (cont-1)